1. Field of the Invention
The present invention relates to a cooling unit for an automotive air-conditioning system and, more specifically, to a cooling unit which reduces the frequency of starting and stopping the compressor thereof when the temperature of the refrigerant in the evaporator is near the frosting point of the evaporator.
2. Description of the Prior Art
A conventional cooling unit for an automotive air-conditioning system is illustrated in FIG. 4. In FIG. 4, there is shown an evaporator 1, a compressor 2 connected to the outlet port of the evaporator 1 with a pipe 3, and a condenser 4 connected to the compressor 2 with a pipe 5 and to the inlet port of the evaporator 1 with a pipe 7 provided with an expansion valve 6. A temperature sensor 8 and a pressure sensor 9 are provided in the pipe 3 near the outlet port of the evaporator 1 to detect the temperature T and the pressure P of the refrigerant. A superheat control means 10 is provided to regulate the flow rate of the refrigerant by regulating the opening of the expansion valve 6 on the basis of the detection signals of the temperature sensor 8 and the pressure sensor 9. The superheat control means 10 has an arithmetic means 11 which determines the degree of superheat on the basis of the temperature T and the pressure P of the refrigerant. The superheat control means 10 regulates the opening of the expansion valve 6 according to the degree of superheat so that the cooling operation is carried out properly. In FIG. 4, indicated at 12 is an AD (analog to digital) converter.
The conventional cooling unit thus constituted, however, has the following problems. For example, when the cooling load on the cooling unit is extremely small, the operation of the compressor 2 is interrupted to prevent frosting of the evaporator 1. Such a manner of operation is designated commonly as cycling driving. Accordingly, when the cooling load increases and decreases alternately and repeatedly, the frequency of interruption of the operation of the compressor 2 at a temperature near the frosting point of the evaporator 1 increases accordingly. Consequently, the load on the comressor 2 increases and unagreeable cooling results.
An invention relating to the cooling unit is disclosed in Japanese Patent Publication No. 54-37421. However, this proposed invention is unable to solve the above-mentioned problems.